The present invention relates to the field of video equipment and visual displays. More particularly, the present invention relates to on-screen displays generated by video equipment, such as set-top terminals and video cassette recorders, in the absence of an incoming video signal.
Modern video equipment including televisions, video cassette recorders (VCRs), and set-top terminals provide users with tremendous access to television programming. In particular, cable and satellite television systems may provide access to hundreds of channels of video programming. A set-top terminal is a box of electronic circuitry connected between a user""s television and a cable or satellite television system for assisting the user in accessing that system.
As the equipment for providing video programming becomes more complicated and offers more features and flexibility to users, the sophistication and programming required to operate such equipment also increases. In fact, some video equipment, particularly VCRs and set-top terminals, now require substantial computing power to optimally process video and audio signals and provide user features such as timed program recording, premium channel de-scrambling, etc.
To support all these features, such video equipment must be readily programmable by users, i.e. receive user input and instructions. To facilitate the programming of such-video equipment as VCRs and set-top terminals, on-screen displays (xe2x80x9cOSDsxe2x80x9d) are commonly used. With an OSD, the equipment being programmed takes temporary control of the television set or monitor to which it is connected in order to display menus, user prompts or echoes of user input to assist the user in properly programming the equipment or accessing equipment features.
In general, to generate a visual display on a display device, such as a television set or monitor, the information for the images to be displayed must be provided as a video signal. This is also true of an on-screen display which is derived from a video signal generated by the video equipment. Conversion of a video signal into a visual image on a display screen requires-the use of a clock signal with a highly accurate frequency. For example, the generation of images on a video display device, e.g., a television set or monitor, may require a clock signal accurate to within 3 to 50 ppm. Some televisions require higher clocking accuracy than others. The first manifestation of an inaccurate clock signal is loss of color, i.e., the displayed image degrades to black and white.
With television signals provided by network broadcasters, or cable or satellite companies, the required clock signal is inherently a part of an analog television signal and is always included as part of a digital television signal. Therefore, when a piece of video equipment takes control of a display device, e.g. a television set, and overrides an incoming video signal to provide an on-screen display, the necessary clock signal with a sufficiently accurate frequency can be derived or borrowed from the incoming video signal.
However, a problem arises when the video equipment needs to provide an on-screen display and no video signal is being received from which to extract a clock signal. This occurs, for example, when the video equipment is connected to a television or monitor on which an OSD can be displayed, but is not connected to a cable or satellite system or to a television antenna, or when the video equipment is connected to a signal source, but is not tuned to and receiving any particular channel carrying a video signal.
When no video signal is provided, video equipment, e.g. a set-top terminal, must generate its own clock signal to support an on-screen display. As noted above, generating a high-quality on-screen display requires a precise clock signal with a highly accurate frequency that is used to construct images from the video signal carrying the information for the on-screen display. Generating such a clock signal can be difficult.
In the past, video equipment with the need to generating a high-accuracy clock signal for supporting an on-screen display has been provided with an internal clock circuit built around a dedicated crystal oscillator. The oscillator is calibrated at the factory to provide a sufficiently accurate clock signal to support an on-screen display. However, the expense associated with the calibration of such oscillators and the tendency of such oscillators to drift out of calibration with age or in response to changing temperatures make this solution less than ideal.
Consequently, a better solution has been developed by General Instrument Corporation of Horsham, Pa. in connection with its set-top terminals. In this solution, a voltage controlled crystal oscillator (VCXO) is used to generate the necessary high-accuracy clock signal. Because the oscillator is voltage-controlled, it requires little or no factory calibration and can be continually controlled to adjust for the effects of initial accuracy, aging or extreme temperatures.
In this arrangement, the control voltage applied to the VCXO is measured and recorded when a video signal is being received and properly displayed. Then, if the video equipment needs to generate a video signal for an on-screen display when no incoming video signal is being received, the recorded voltage value is retrieved from memory, and a corresponding voltage is applied to the VCXO to generate the required clock signal for supporting the on-screen display.
In a specific embodiment, an additional A/D converter (1 bit pwm and comparator) is used to measure the VCXO control voltage when the VCXO is locked to the timing of the incoming video signal. This control voltage is then used to adjust the control voltage of the VXCO to the same value when no video signal is being input to the equipment.
This arrangement, however, requires that the set-top terminal incorporate the electronic means for making a measurement of the voltage on the VCXO during reception and display of an incoming video signal. It would be advantageous if this need to measure a voltage value could be eliminated. Consequently, there is a need in the art for a method and apparatus of providing a circuit that can generate a clock signal with a sufficiently accurate frequency to support an on-screen display without requiring factory calibration, being susceptible to aging or temperature or requiring a previous voltage measurement.
It is an object of the present invention to meet the above-described needs and others. Specifically, it is an object of the present invention to provide a method and apparatus for controlling a VCXO in the absence of an incoming video signal to produce a clock signal with a sufficiently accurate frequency to support an on-screen display, particularly without requiring a previous voltage measurement.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the invention. The objects and advantages of the invention may be achieved through the means recited in the attached claims.
To achieve these stated and other objects, the present invention may be described as a clock circuit for outputting a clock signal for video reconstruction in the absence of an incoming video signal. The present invention may be embodied in a clock circuit that includes a control logic circuit with a phase locked loop for receiving an incoming video signal and phase locking to the clock signal component within the incoming video signal. The control logic circuit then outputs a control signal based on the phase lock to control the output of a voltage controlled oscillator.
The clock circuit also includes a circuit for recording control data from the control signal during the phase lock. The recorded control data is then used by the clock circuit to control the voltage controlled oscillator in the absence of an incoming video signal. Preferably, the circuit for recording control data includes a processor and a memory unit. The processor may monitor the incoming video signal and automatically provide the recorded control data from the memory unit in the absence of the incoming video signal.
The voltage controlled oscillator outputs a clock signal with a sufficiently accurate frequency to support an on-screen display in the absence of the incoming video signal from which a clock signal can be obtained. A voltage signal generator is preferably used to receive the control signal from the control logic circuit and to drive the voltage controlled oscillator in accordance with that control signal.
The present invention may be applied regardless of whether the incoming video signal is analog or digital. Where the incoming video signal is a digital signal, the phase locked loop locks to program clock reference data in the incoming video signal. Where the incoming video signal is an analog signal, the phase locked loop locks to the horizontal frequency of the incoming video signal.
The present invention also encompasses a corresponding method of producing a clock signal for video reconstruction in the absence of an incoming video signal. The method includes the steps of retrieving previously recorded control data for a voltage controlled oscillator which was output by a control logic circuit with a phase locked loop and recorded when the phase locked loop was locked to a clock signal component within an incoming video signal.
The method further includes the steps of:
(1) recording the control data that is output by the control logic circuit with a phase locked loop when the phase locked loop is locked to a clock signal component within an incoming video signal; and
(2) monitoring the status of the incoming video signal so as to initiate the retrieval of the recorded control data when the incoming video signal is not present.
The present invention further encompasses an embodiment comprising: a voltage controlled oscillator for outputting a clock signal and a control logic circuit for outputting a control signal to control the voltage controlled oscillator. The control logic circuit compares the clock signal with a reference clock signal, when present, and adjusts the control signal in accordance with the comparison. The clock signal is not necessarily the same frequency as the reference clock signal. Moreover, the clock signal being adjusted does not interfere with the display of a video signal using its own clock signal component. The reference clock signal may be taken from an in-band or out-of-band digital data recovered clock.
A recording circuit records the control data of the control signal during the comparison. The recorded control data is then used by the clock circuit to control the voltage controlled oscillator in the absence of an incoming video signal.
Finally, the present invention encompasses a corresponding method of generating a clock signal for video reconstruction in the absence of an incoming video signal. The method includes using stored control data to control a voltage controlled oscillator to output a clock signal, where the stored control data was taken from a control signal output by a control logic circuit while the control logic circuit compared the clock signal from the voltage controlled oscillator with a reference clock signal and output the control signal in accordance with said comparison.